Emit OpenTelemetry tracing

OpenTelemetry has become the de-facto standard for distributed, cross-platform tracing; it is being rapidly adopted both inside the .NET ecosystem and outside. OTel defines semantic conventions for how a database client traces information; [these specifications](https://opentelemetry.io/docs/specs/semconv/database/database-spans/) are currently in experimental state, but stabilization is likely start happening soon.

There is a separate instrumentation library, [OpenTelemetry.Instrumentation.SqlClient](https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Instrumentation.SqlClient/README.md#steps-to-enable-opentelemetryinstrumentationsqlclient), which AFAIK uses the DiagnosticSource instrumentation already emitted by SqlClient to then emit OpenTelemetry tracing.

However, ideally SqlClient would emit this tracing data directly, without requiring a 3rd-party library.